The field of this invention relates to a radio frequency (RF) transmitter architecture, an integrated circuit device, a wireless communication unit and a method therefor. The invention is applicable to, but not limited to, a method and apparatus for performing digital pre-distortion of a signal for transmission over an RF interface.
Advances in the deep sub-micron CMOS (Complimentary Metal-Oxide Semiconductor) process have led to digital circuits becoming smaller and more power efficient. However, analogue circuits do not scale well with the deep sub-micron CMOS process. It is therefore desirable for devices such as radio frequency (RF) transmitters to remove as many analogue components as possible with the assistance of digital signal processing algorithms.
Conventional RF transmitters use linear power amplifiers (PAs). Accordingly, the power efficiency of such conventional RF transmitters is usually very low due to the low efficiency of the linear PAs used therein. Switch-mode PAs have very high efficiency in comparison, which make such switch-mode PAs an attractive alternative to conventional linear PAs within RF transmitters.
Thus, an RF transmitter that can utilize switch-mode PAs through the assistance of digital processing algorithms to reduce the size and improve power efficiency is highly desirable. However, switch-mode PAs normally exhibit a highly non-linear input-output relationship due to, for example, load interactions between the I-path and Q-path of PAs in an in-phase/quadrature (IQ) transmitter architecture. Accordingly, switch-mode PA architectures require non-linear compensation. In addition to such non-linear compensation of PAs, such IQ transmitter designs are also required to compensate for imbalance between the I and Q paths due to, for example, local oscillator (LO) mismatch as well as I-section PA and Q-section PA mismatch.
Digital polar transmitters are another type of known transmitter design that also achieves high efficiency by operating the PAs in compressed mode. A problem with digital polar transmitter designs is that, due to the inherent bandwidth expansion characteristics of converting IQ signals into amplitude modulation (AM) and phase modulation (PM) in a polar architecture, they are only suitable for narrowband modulated signals.
To accommodate a need for increasingly higher throughput on the go, some wireless standards such as WCDMA (wideband code division multiple access), LTE (Long Term Evolution), IEEE 802.11 for WLANs (wireless local area networks), etc., have been proposed to support modulation signals with peak-to-average power ratio (PAPR) and bandwidth from 5 MHz to 20 MHz, or to even wider bandwidths, such as 160 MHz in IEEE 802.11ac. It is therefore also desirable to have a common transmitter architecture that can support multi-mode and multi-bandwidth operation while simultaneously achieving low current consumption and small form factors.
Thus, a need exists for an improved RF transmitter architecture, and method of performing digital pre-distortion and digital compensation technology of a signal for transmission over an RF interface.